The IEEE 802.11 standard provides a common medium access control (MAC) layer that is adapted to provide a variety of functions, which support 802.11 based wireless local area networks (WLANs). The MAC layer is adapted to facilitate and manage communication between access points (APs) and stations (STAs) over a shared wireless communication channel. The MAC layer is adapted to handle a plurality of functions such as scanning, authentication, association, power saving and fragmentation. Optional functions provided by the 802.11 MAC may comprise encryption and RTS/CTS handshaking.
The 802.11 standard comprise a passive scanning mode and an active scanning mode. In the passive scanning mode, a wireless station such as a wireless transceiver or NIC searches for service by listening for access points on a channel or on a succession of channels. No transmissions are made by a wireless station which is passively scanning. Within the 802.11 standard, passive scanning is defined as the mandatory scanning mode and active scanning is defined as an optional mode. In active scanning, each wireless transceiver or NIC sends probe frames which are intended to elicit a probe response frame in order to scan individual channels to locate access points. The best access point for tentative association is determined from the physical properties of the signals received at the wireless station from each of the access points, combined with various other information received during the scanning operation, such as access point supported rates, QOS capabilities, current load factor and the support of other features. An access point may periodically broadcast a beacon frame, which may be received by the wireless transceiver or STA receives during scanning. The beacon frame may comprise signal strength information for corresponding APs, as well as access point specific information such as service set identifier (SSID), and data rates that are supported by the access point. The wireless STA may determine which AP to connect based on the access point specific information received from one or more access points. During the optional active scanning mode, a wireless STA may broadcast a probe frame, and all access points receiving the probe frame may respond with their corresponding specific information such as SSID, signal strength, and supported data rates. Active scanning allows a wireless STA to receive a faster response, on average, from one or more access points, instead of having to wait for transmission of a beacon frame. One drawback with active scanning is that it imposes additional network overhead since probe frames are transmitted and response frames are received form responding APs. Additionally, the wireless STA performing the active scanning may interfere with the normal traffic of the network because the scanning STA has switched to the current channel with little information regarding the current channel state.
The 802.11 standard provides open system authentication methodology and a shared key authentication methodology for proving an identity of a networking entity such as a wireless STA. In the 802.11 standard, the open system authentication methodology is specified as being mandatory, while the shared key authentication methodology is specified as being optional. With open system authentication, a wireless STA may initiate authentication by sending an authentication request to an access point. In response, the access point may reply with an authentication response, which may approve or deny authentication. An approval or a denial of authentication may be indicated in a status code field within a frame. For optional shared key authentication, authentication may be effected based on whether an authenticating device such as a wireless STA possesses an appropriate wired equivalent privacy (WEP) key. In this regard, the wireless STA may send an authentication request to an access point and the access point may respond by placing challenge text within a response frame, which is sent to the wireless STA. The wireless STA is configured to encrypt the challenge text using its WEP key and the encrypted challenge text is then transmitted from the STA to the access point. Upon receiving the encrypted challenge text, the access point is adapted to decrypt the encrypted challenge text and compares it to the initial text. If the comparison of the decrypted text and the initial text indicates a match between the two, then the access point assumes that the wireless STA possesses the correct wired equivalency privacy key. As a result, the access point will send an authentication frame to the wireless STA, which indicates a service approval if there is a match or a service denial if the match fails.
After a wireless STA accesses the wireless medium and is authenticated, the wireless STA has to associate with the access point prior to start of data communication. Association allows tasks such as synchronization, and exchange of important information between an access point and a wireless STA. For example, during association, associated data rates may be communicated from an access point to a wireless STA. A wireless STA may be adapted to initiate association by communicating an association request comprising information such as supported data rates, optional capability support, security capability support, other optional feature support and SSID information. In response, an access point may communicate an association response frame comprising an association identifier (ID) and other access point specific information. Once the wireless STA and access point complete association, the wireless STA may then start communication of data fames with the access point.
The 802.11 standard provides an optional power save mode that may be enabled or disabled by a user, if available. If implemented, the power save mode allows a user to enable the wireless STA to turn ON or turn OFF its wireless transceiver as appropriate, in order to conserve battery power. For example, when it is not necessary for the wireless transceiver or STA to communicate information, the wireless STA may turn OFF its wireless transceiver. In instances when the power save mode is turned ON, a wireless STA may notify an access point of the possibility that it may enter a sleep state. A status bit in the header of each frame may be utilized to indicate the power save mode of the wireless STA. In this regard, the power save mode may be turned ON when this bit is asserted and turned OFF when this bit is deasserted. An access point is configured to keep track of each wireless station that indicates its intent to enter or exit the power save mode. This allows the access point to buffer packets for those wireless STAs that have indicated the possibility that they may enter sleep state while they are in sleep mode and to avoid buffering packets for those wireless STAs that have indicated their intent to exit (or not enter) sleep mode. The STAs which have entered sleep mode may periodically wake up from sleep state and check whether the access point has buffered data or whether new data is waiting to be delivered.
The IEEE 802.11 protocol provides support for two different medium access control (MAC) mechanisms that may be utilized for transporting asynchronous and time bounded services. The first mechanism is distributed coordination function (DCF) and the second is point coordination function (PCF). The distributed coordination function utilizes best effort for facilitating communication of information in which access devices with information to transmit have an equal opportunity to transmit information. The point coordination function maybe utilized to communicate time sensitive or latency sensitive information. In this regard, the point coordination function utilizes a polling mechanism, which may be controlled by an access point (AP) acting in the role of a Point Coordinator (PC).
Before transmitting frames, a station is required to first gain access to the shared wireless medium. The 802.11 standard defines a distributed coordination function (DCF) type of medium access and a point coordination function (PCF) of medium access. The DCF type of medium access is mandatory and it utilizes carrier sense multiple access with collision avoidance (CSMA/CA) protocol. DCF allows a plurality of wireless STAs to contend for access to the wireless medium when the wireless STAs attempt to send frames. The wireless STAs may utilize a binary back off mechanism to provide a fair medium access mechanism. Accordingly, a wireless STA will back off for a random amount of medium idle time before attempting to access the medium.
The MAC layer utilizes a network allocation vector (NAV) to ensure fair access to the medium. The NAV is a counter, which resides at each wireless station and represents the amount of time that a frame or sequence of frames will require to send data contained therein. In order to allow fair access to the medium, the MAC Layer checks the value of the network allocation vector (NAV). A wireless STA is allowed to send a frame when its NAV is zero and any backoff has been completed. A station is required to determine the amount of time that it will need to transmit the current frame plus any subsequent frames which are part of the same frame sequence, based on the length of the frames and the frames' data rates, before it is allowed to transmit a frame. The station will place this determined time in a duration field in the header of frames that are to be transmitted. When the wireless STAs receive the frame, the time is acquired from the duration field of the received frame and utilized to determine a corresponding value for their NAVs.
The random back off timer employed in DCF may be utilized by a STA to determine or detect whether the medium is accessible or busy. If it is determined that the medium is busy, a STA has to wait for a randomly generated period of time before another attempt is made at accessing the medium. This ensures a fair access mechanism and reduces the probability of multiple stations sending data on the medium at the same time. The random delay imposed by the back off prevents a plurality of wireless STAs from simultaneously sensing the medium as being idle at the conclusion of a singular transmission and subsequently attempting transmission at the same time, which would result in collisions. Accordingly, the random back off timer significantly reduces the number of collisions and hence the number of retransmissions and this is particularly important as the number of active wireless STAs increases.
A wireless STA may not listen for collisions while it is transmitting data because it cannot have its receiver turned ON while it is transmitting data. This means that a receiving wireless STA has to send an acknowledgement (ACK) whenever no errors are detected in a received frame. If a transmitting STA does not receive an ACK after a determined period of time has elapsed, the transmitting STA automatically assumes that a collision has occurred and will retransmit the frame on it own accord. The 802.11 standard provides time-bounded delivery of data frames via the optional point coordination function (PCF). In the optional point coordination function, an access point may grant access to the medium on a per station basis via polling during a contention free period. In this regard, a wireless STA has to be polled before it is allowed to transmit frames. PCF traffic may be communicated between alternate contention or DCF periods. In this regard, an access point may poll wireless STAs based on a polling list, and switches to a contention period during periods in which the wireless STA utilize DCF. This may permit a synchronous operating mode as well as an asynchronous operating mode. For example, the synchronous operating mode may be utilized to support video based applications and the asynchronous operating mode may be utilized to support browsing or messaging applications.
A wireless STA is adapted to encrypt the payload of each frame using a common WEP key prior to transmission of each frame. A receiving wireless STA or access point, upon receiving the encrypted frame, will decrypt the received encrypted frame using the common WEP key. There is a plurality of different size common WEP keys that are available and each is adapted to provide varying strengths of encryption. Additional security schemes are also supported by the protocol.
Handshaking signals, which comprise request-to-send (RTS) and clear-to-send (RTS/CTS) are utilized by an access point or STA to control access to, and use of, the wireless medium by RTS/CTS enabled STAs. A STA may establish a maximum frame length and whenever the maximum frame length is exceeded, RTS/CTS may be automatically utilized. Whenever a wireless STA activates RTS/CTS handshaking mechanism, the wireless STA will transmit an RTS frame to an access point or another STA before it transmits a data frame. In response, the access point or other STA will transmit a CTS frame, which indicates that the wireless STA may transmit the data frame. With regards to a CTS frame, an access point or STA may modify the duration value from the duration field within the frame header of the RTS frame and place this modified value into the duration field within the frame header of the CTS frame. This will bar other stations from transmitting until the wireless STA that initiated the RTS transmits the data frame has completed transmitting the data frame and has had an opportunity to have received the ACK frame.
During transmission, it may be more efficient to transmit smaller segments of information rather than larger segments of data. These smaller segments of information may be referred to as fragment. For example, a frame comprising L2 header information, L3 header information, L4 header information, ULP information and payload data may be segmented into a plurality of segments in which all the L3, L4, L5 headers are in a single fragment, the ULP information and a portion of the payload data may be in another fragment, and a remaining portion of the payload data may be fragmented into a plurality of other fragments. If the fragmentation occurs at the 802.11 layer, in compliance with the standard, a transmitter will not transmit these fragments out-of-order. In this regard, the transmitter may not begin the transmission of a subsequent (n+i) fragment, where i is greater than 1, until the nth or prior fragment has been successfully transmitted. Accordingly, out-of-order (OOO) fragments would not occur under such circumstances.
The standard distributed coordination function for medium access may be inefficient in terms of bandwidth utilization, especially at higher physical layer (PHY) speeds, for example, 54 Mbps or higher. The DCF may be adapted to solve problems such as network congestion and high packet error rate (PER) typically associated with some wireless links. The DCF may also exponentially increase backoff and positive acknowledgments (PACKs). The backoff time for each MAC protocol data unit (MPDU) may increase exponentially for retransmissions and the PACK for each MPDU may render bandwidth utilization inefficient at high physical layer (PHY) speeds. The RTS/CTS mechanism when utilized in conjunction with regular DCF, may diminish efficiency even more, and as a result, may be rarely utilized. For example, in a case where no RTS/CTS is utilized, transmitting a 1500 byte frame including MAC header at 54 Mbps takes 248 μs. The sum of the average backoff, PACK and the short interframe space (SIFS) takes 130 μs, when PACK is transmitted at 24 Mbps. The overhead air time may be more than half the data air time.
The distributed coordination function is not the most bandwidth efficient transport mechanism. The IEEE 802.11 standard defines a bursting method for MAC protocol data units (MPDUs), called fragmentation. In this regard, MAC service data units (MSDU) may be fragmented at the MAC level to a number of smaller MPDUs. The individual MPDUs comprising one MSDU may be transmitted in a “burst”, in which the interframe spacing is a SIFS, and PACK frames follow the transmission of each fragment. Hence, a typical frame exchange sequence under fragmentation would be DATA-SIFS-PACK-SIFS-DATA-SIFS-PACK, for example, with an optional RTS/CTS exchange in the beginning. However, fragmentation was defined in the 802.11 standard as a means to combat unreliable wireless links having high packet error rates (PER). In reliable wireless links, that is, those with low packet error rates, fragmentation may decrease MAC efficiency, since it introduces MAC headers on each MPDU, and SIFS intervals between MPDUs and PACKs.
In order to address various issues dealing with quality of service (QoS) such as guaranteed delivery of a particular QoS and MAC efficiency, more efficient bandwidth allocation and usage mechanisms are required. The IEEE 802.11e draft standard defines block acknowledgement policies that eliminate the need for individually transmitted acknowledgements (PACKs) for each MPDU. This block acknowledgement scheme allows multiple fragments and/or frames to be sent at the MAC layer, without having to issue an individual positive acknowledgement for each fragment. However, this block PACK mechanism introduces extra overhead comprising block PACK request frames and block PACK response frames. Under the block acknowledgement scheme, a transmitter may send a parameterized number of frames before a block PACK response is received, which acknowledges receipt of the frames in the blocks. MAC layer acknowledgement mechanisms notwithstanding, there are instances when out-of-order TCP segments may be transmitted and accordingly, this will lead to out-of-order fragments. The IEEE 802.11e draft standard also defines contention free periods of time allocated to specific devices, where frames may be transmitted with a SIFS period of separation, rather than the previously described back off separations between frames transmitted by a single wireless STA. The mechanism for this allocation may be complex, and may include overhead for the polling mechanisms involved.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.